The present invention relates to a method of periodically heating a product separator of a plant for producing phthalic anhydride (PA) or maleic anhydride (MA), wherein a liquid heat transfer medium is indirectly heated by means of steam in a heat exchanger (heater for the heat transfer medium) and the liquid medium is circulated through the product separator to indirectly heat and melt the PA or MA product deposited on cooling surfaces.
In the conventional separation of the raw PA or MA product, the gas coming from the producing plant is cooled and the product is deposited mainly by desublimation on the outside of an indirectly cooled heat exchanger. The deposited raw product is discharged after it has been melted. The liquid heat transfer medium is mostly the same as that used in the cooling phase and usually consists of heat transfer oil and is conducted through the heat exchanger at a temperature which exceeds the melting temperature of the product. Outside the product separator the liquid heat transfer medium is reheated in the heater for the heat transfer medium; that heater is heated with steam. A method of that kind has been described in European Patent Specification No. 35 173. In that known method the strong fluctuations of the heat demand are compensated in that the liquid heat transfer medium is conducted through a heating circuit, which includes a reservoir, and liquid heat transfer medium is periodically taken from that reservoir.